Rewritable, self-erasing ‘paper’ developed

RESEARCHERS in China have developed a type of rewritable, self-erasing, paper-like material which is made of low-cost substances and could help to reduce paper waste.

Large quantities of printed paper materials are produced every day, whether in offices, as posters and banners, or as signs, but many are only used for a short period of time before being discarded. While recycling the paper can reduce emissions and energy and water use, it would be better to avoid the waste in the first place. Dairong Chen and his colleagues at Shandong University’s chemistry and chemical engineering school, say their new material, made of cheap polyvinylpyrrolidone (PVP) polymer and tungsten oxide (WO₃), could do just that. They successfully erased and reprinted sheets of the material, a photochromic membrane, more than 40 times, without losing resolution.

WO₃ is colourless in normal conditions (oxidising conditions, including ambient air), but when exposed to an electric field or UV light, it changes to a deep blue colour. When exposed to reducing conditions, for example oxygen or ozone, the WO₃ returns to its colourless state. It is this property that the Shandong researchers have taken advantage of. They disperse amorphous WO₃ into a solution of dimethylformide and PVP and produce sheets of fibrous membranes through an electrospinning process – making the ‘paper’. The fibres are randomly orientated and produce a smooth surface.

To ‘print’ on the membranes, the researchers expose them to UV light, and use stencils so that only exposed parts turn blue. The process takes around two minutes, depending on how much WO₃ has been incorporated into the membrane. The image fades in ambient conditions over around two days, or it can be erased in around half an hour with exposure to heat or ozone. The length of time the print remains on the membrane in ambient conditions can be tuned. For example, adding a small amount of polyacrylonitrile can make the print last up to ten days.

Chen and the team also tested their system on fabric, to further demonstrate its suitability for reprintable clothes which could be used for temporary advertising and promotion purposes or for sports meetings. They incorporated a reprintable pocket into a lab coat, with the membrane sewn into it, which they then printed with a blue image. The image faded over a couple of days but could be reprinted multiple times.

The membranes are superior to previous versions of reprintable materials, as they do not contain toxic or expensive organic dyes, such as viologens and spiropyrans, and exhibit relatively fast colour-switching. They can also be erased and reprinted many more times than other reprintable materials.

“As-formed photochromic membranes are low-cost, environmentally benign and easy for large-scale production, indicating their great potential as flexible, rewritable material for practical usage,” conclude the researchers.

Applied Materials & Interfaces http://doi.org/bsts